1. Field of the Invention
The present invention relates to an apparatus for and method of generating orthogonal frequency division multiplexed signal, a demodulator, and a communication apparatus.
2. Description of the Related Art
Commercializing of the OFDM (Orthogonal Frequency Division Multiplexing) system is promoted as a modulation method of digital communication.
An example of a system to which OFDM is applied is the EUREKA-147 SYSTEM. Generally, this is referred to as DAB (Digital Audio Broadcasting) or the EUREKA-147 DAB system. This EUREKA-147 DAB system was approved by ITU-R (International Telecommunication Union-Radio communication sector) on November 1994 as System-A and becomes an international standard. This standard has been issued as xe2x80x9cETS 300401xe2x80x9d.
In the EUREKA-147 DAB system, each sub-carrier in the OFDM is subjected to phase modulation. As a phase modulation system, is employed the differential QPSK (Quadrature Phase Shift Keying) system, in which main data is transmitted by phase differences of respective sub-carriers between an OFDM symbol duration and the just preceding OFDM symbol duration.
In the EUREKA-147 DAB system, since data is transmitted by phase differences of respective sub-carriers between OFDM symbols, a demodulation means in a receiver requires phase information that becomes a reference. For this purpose, phase reference symbols are periodically inserted into an OFDM signal being transmitted. Each phase reference symbol is constituted by sub-carriers having predetermined reference phases.
Further, in demodulating operation at receiving, each OFDM symbol duration should be specified in order that each OFDM symbol duration is converted onto a frequency axis to extract sub-carrier components on the frequency axis. To that end, in the EUREKA-147 DAB system, a symbol for coarse synchronization, called a null symbol, is inserted just before a phase reference symbol.
The null symbol is a xe2x80x9cnullxe2x80x9d signal having no sub-carrier for transmitting main data. Usually, on the receiving side, a null symbol duration is specified from change of an envelope of the received signal, in order to decide roughly each OFDM symbol duration.
In the EUREKA-147 DAB system, it is possible to optionally superimpose a TII (Transmitter Identification Information) signal on a null symbol duration to identify a transmitting station. The TII signal is transmitted by transmission of only some predetermined sub-carriers out of the sub-carriers of an OFDM symbol. In that case, a null symbol duration is not a null signal but has a waveform of slight amplitude. However, it is suppressed to relatively small amplitude as compared to the amplitude of other OFDM symbol durations transmitting the phase reference symbol or main data.
Taking an example of the mode 2 of the EUREKA-147 DAB system, arrangement of the sub-carriers on the frequency axis in a null symbol duration will be described in the case that a TII signal is added. In the mode 2 of the EUREKA-147 DAB system, information is transferred by 384 sub-carriers.
FIG. 1 shows an arrangement of the sub-carriers on the frequency axis in a null symbol duration in the case that a TII signal is added. As shown in FIG. 1, a TII signal is constituted by some pairs of adjacent sub-carriers of predetermined frequencies in accordance with an identification code defined for each transmitting station. As compared to other OFDM symbols, the number of the sub-carriers is very small. Accordingly, when observed, it has such a waveform that a signal of relatively small amplitude is superimposed on a null symbol duration.
As described above, when a TII signal is superimposed on a null symbol duration, the null symbol duration is actually not a null signal but has a waveform of slight amplitude. Thus, it is possible that superimposition of a TII signal on a null symbol duration makes detection of the null symbol difficult in a receiver. For example, in a location of bad receiving conditions, it is difficult to discriminate between a null symbol and the other OFDM symbols. Further, depending on receiving conditions, a transmitting station expressed by a TII signal may become difficult to be discriminated.
Such situation is not limited to the transmission of a TII signal, and it is possible that similar difficulty is caused when a null symbol duration is utilized for transmitting other data, in addition to data transmitted by phase modulation.
An object of the present invention is to provide an apparatus for and method of generating an orthogonal frequency division multiplexed signal, as well as a demodulator and a communication apparatus, in which, in addition to a first data transmitted by phase modulation of sub-carriers, a second data can be transmitted without using a null symbol duration.
A first orthogonal frequency division multiplexed signal generator according to the present invention is for generating a signal that is multiplexed into two or more sub-carriers by OFDM, and comprises:
a data sequence conversion means for arranging first data to be transmitted, in a predetermined order correspondingly to two or more sub-carriers orthogonal to each other on a frequency axis;
a power changing means for changing powers of the sub-carriers in accordance with second data to be transmitted which is independent from the first data;
an inverse discrete Fourier transform means for generating a time-base waveform by synthesizing the two or more sub-carriers changed by the power changing means; and
an orthogonal modulation means for performing orthogonal modulation on a real axis signal and an imaginary axis signal which are orthogonal to each other and generated by the inverse discrete Fourier transform means, to synthesize the real axis signal and the imaginary axis signal.
In that case, the power changing means may change the powers of the sub-carriers to one of 2 to the s-th power levels in accordance with s bit values of the second data. The power changing means may change powers of a plurality of sub-carriers in accordance with the same second data. The power changing means may change powers of predetermined sub-carriers out of the two or more sub-carriers. The predetermined sub-carriers may be determined in advance corresponding to a TII signal. Further, the power changing means may change powers of sub-carriers in a plurality of OFDM symbol duration in accordance with the same second data.
A second orthogonal frequency division multiplexed signal generator according to the present invention comprises: a phase modulation means for modulating phases of the sub-carriers in accordance with first data; and an amplitude modulation means for modulating amplitudes of the sub-carriers in accordance with second data.
In that case, the amplitude modulation means may superimpose the same second data a plurality of times on the same OFDM symbol. The amplitude modulation means may perform amplitude modulation by reversing polarities for a pair of sub-carriers with each other.
A third orthogonal frequency division multiplexed signal generator according to the present invention comprises:
a data sequence conversion means for arranging first data to be transmitted, in a predetermined order correspondingly to two or more sub-carriers orthogonal to each other on a frequency axis;
a delay means for delaying the first data for a predetermined time to output as second data;
a power changing means for changing powers of the sub-carriers in accordance with the second data;
an inverse discrete Fourier transform means for generating a time-base waveform by synthesizing the two or more sub-carriers changed by the power changing means; and
an orthogonal modulation means for performing orthogonal modulation on a real axis signal and an imaginary axis signal which are orthogonal to each other and generated by the inverse discrete Fourier transform means, to synthesize the real axis signal and the imaginary axis signal.
In that case, when a minimum unit for transmitting the second data is s bits, the power changing means may change a power spectrum of the sub-carriers according to 2 to the s-th power levels. Further, when a minimum unit for transmitting the first data assigned to the sub-carriers is p bits, the power changing means may select the s bits so that s is equal to or more than p, to transmit the second data.
A fourth orthogonal frequency division multiplexed signal generator according to the present invention comprises:
a data sequence conversion means for generating complex data corresponding to two or more sub-carriers orthogonal to each other on a frequency axis, based on first data;
a power changing means for changing power of the complex data in accordance with second data;
an inverse discrete Fourier transform means for performing inverse discrete Fourier transform on the complex data changed by the power changing means; and
an orthogonal modulation means for performing orthogonal modulation on a real axis signal and an imaginary axis signal which are orthogonal to each other and generated by the inverse discrete Fourier transform means, to synthesize the real axis signal and the imaginary axis signal.
A demodulator according to the present invention is one for demodulating an orthogonal frequency division multiplexed signal. The demodulator comprises:
a quadrature detection means for performing quadrature detection on the multiplexed signal to obtain a first detection axis signal and a second detection axis signal which are quadrate to each other;
a discrete Fourier transform means for sampling respective time-base waveforms of the two detection axis signals at a predetermined sampling frequency, and for performing discrete Fourier transform on these sampled data to obtain two or more metrics distributed in a frequency domain;
a second data extracting means for extracting second data based on powers of the metrics obtained by the discrete Fourier transform means; and
a first data extracting means for extracting first data from phase changes of sub-carriers for each symbol, obtained by the discrete Fourier transform means.
The demodulator may further comprise an averaging means for averaging the auxiliary data extracted by the auxiliary data extracting means, for a plurality of OFDM symbol duration.
A method of generating an orthogonal frequency division multiplexed signal according to the present invention is one for generating a signal that is orthogonal-frequency-division-multiplexed into two or more sub-carriers. A first method of generating an orthogonal frequency division multiplexed signal comprises the step of modulating a plurality of data given independently of one another, using different modulation systems.
A second method of generating an orthogonal frequency division multiplexed signal comprises the steps of: generating delayed data from given data by delaying the given data for a predetermined time; and performing independent modulation on the delayed data using a different modulation system from one applied to the given data.
A first storage medium according to the present invention is one storing an orthogonal frequency division multiplexed signal generating program for generating a signal that is orthogonal-frequency-division-multiplexed into two or more sub-carriers. The program comprises the steps of:
arranging first data in a predetermined order correspondingly to two or more sub-carriers orthogonal to each other on a frequency axis;
changing powers of the sub-carriers in accordance with second data given independently of the first data; and
performing inverse discrete Fourier transform on the two or more sub-carriers changed in their power, to generate a time-base waveform.
A second storage medium according to the present invention is one storing a demodulation program for demodulating an orthogonal frequency division multiplexed signal. The program comprises the steps of:
receiving two detection axis signals detected by orthogonal detection and sampling respective time-base waveforms of the two detection axis signals at a predetermined sampling frequency;
performing discrete Fourier transform on the respective sampled data to obtain respective phase changes of sub-carriers for each symbol and two or more metrics distributed in a frequency domain;
extracting first data from the respective phase changes of the sub-carriers for each symbol; and
extracting second data based on powers of the two or more metrics.
A communication apparatus according to the present invention is one for communicating using an orthogonal frequency division multiplexed signal, and comprises:
a transmission means for performing orthogonal frequency division multiplex modulation on a carrier in accordance with data given by an inputted signal, to transmit a multiplexed signal; and
a receiving part for performing orthogonal frequency division multiplex demodulation on the received signal to detect the modulation data and for outputting the signal given by the modulation data. The transmission means is provided with one of the above-described first through fourth orthogonal frequency division multiplexed signal generator. Further, the receiving means is provided with the above-described demodulator.